The present invention relates to a method and apparatus for recording information signals into a magnetic record medium used in a device for magnetic recording and reproduction with high recording density and large capacity, a master information carrier to be used for the recording method, and a method for making the master information carrier.
Recently, a magnetic recording and reproduction apparatus has been increasing recording density to realize small size and large capacity. Especially, in the field of a hard disk as a typical magnetic recording device, an areal recording density of more than one gigabit per square inch is already available on the market, and an areal recording density of ten gigabits per square inch is expected in a couple of years. The technology proceeds with a rapid pace.
One of the primary factors that has enabled such high recording density is the increasing linear recording density, due to improvements of medium properties, head-disk interface performance, and new signal processing method such as xe2x80x9cpartial responsexe2x80x9d. However, in recent years, the rate of increase in track density exceeds that of linear record density, and thus becomes a primary factor of the increasing areal recording density. Practical use of a magneto-resistive type head, which is superior to a conventional inductive type head in read-back signal performance, has contributed to the progress in the track density. It is possible at present to read a signal from a track whose width is only a few microns with good S/N ratio by practical use of the magneto-resistive type head. On the other hand, it is expected that a track pitch will reach the sub-micron range in the near future along with further improvement of the head performance.
A tracking servo technique is important for the head to read a signal with high S/N ratio by tracing such a narrow track. For example, a conventional hard disk has areas that are located at predetermined angles over 360 degree and in which information such as a tracking servo signal, address and clock signal are written. In this specification, preformat writing or prewriting of such an information signal is called a xe2x80x9cpreformat recordingxe2x80x9d. A head can trace a track by reading such information in predetermined intervals, and monitoring and correcting the head position.
The above mentioned tracking servo signal, address and clock signal become reference signals for the head to trace a track precisely. Therefore, precise record positions are required for these information signals. Current preformat recording into a hard disk is usually performed by magnetic heads placed in the hard disk drive by using a special servo track writer after installing the disk and the head into the drive. In this case, a required accuracy of the track position for writing is achieved by precisely controlling the position of the head incorporated in the drive by using an external actuator equipped to the servo track writer.
Such a preformat recording of servo signal address information and clock signal is performed similarly for large capacity flexible disks or disk cartridges, which are removable disk media seen in the market recently, by using a magnetic head and a servo writer. These media are removable, so they can be compatibly used by other drives. Therefore, it is not always required to perform the preformat writing by the heads of each drive after incorporating the heads into the drive though it is required for a normal hard disk. However, these removable disks are similar to normal hard disks from the viewpoint that the preformat writing is performed by precisely controlling the position of the head by using an external actuator equipped to the servo track writer.
However, in the present preformat recording of servo signal, address information and dock signal, there are the problems described below.
The first problem is that writing with the magnetic head is a linear recording relying on relative movement between the magnetic head and the recording medium. This means that a long period is required for preformat writing by the above-mentioned method, while precisely controlling the position of the magnetic head with a servo track writer. Moreover, because the servo writer is expensive, the cost for preformat writing is high
This problem becomes even more serious as the areal recording density increases. This is not only caused by an increase of tracks in radial direction. As the track density increases, a higher precision is required for the head positioning and as a result, servo areas, in which the tracking servo signal and other signals are recorded, have to be provided with smaller angular distances between them over 360 degrees. Moreover, the address information to be written as the preformat data increases as the recording density increases. Thus more time and cost are required for writing more information signals as the record density becomes higher.
A smaller size for magnetic disks is expected to be the trend on the market. However, large disks of 3.5 or 5 inch size are still in demand. These large disks require more information signals to be written for the preformat than the small disks. The necessary time for preformat writing influences the cost effectiveness of such large disks.
The second problem is that a space between the head and a medium or a diffusive recording magnetic field due to a pole shape of the record head does not make a steep magnetic transition at track edges where the preformat data is written. Relative movement between the magnetic head and a medium is indispensable in writing with the head, so some space is necessary between the head and the medium for interface performance between them. A conventional magnetic head usually has two elements for writing and reading. A pole width at a trailing edge of the head corresponds to a record track width, and a pole width at a leading edge is several times larger than that at the trailing edge.
The above two phenomena may be a factor for causing the diffusive recording magnetic field to fringe over the preformatted record track width, resulting in the magnetic transition at track edges not being steep or erased areas appearing on both sides of a track. In current tracking servo techniques, the head position is detected by a change in read signal amplitude when the head misses a track. Therefore, as in the process of reproducing the data signal recorded between the servo tracks, the system requires not only a high S/N ratio of a read signal when the head traces a track correctly, but also a steep off-track performance, in which the read signal amplitude changes steeply as the head misses the track. If the magnetic transition is not steep enough at an edge of a track where the preformat is written, it is difficult to realize a precise tracking servo performance that will be required for a sub-micron track recording in the future.
As a solution of the first of the two problems mentioned above, a duplicate record technique of a tracking servo signal or other signals by using a magnetic transfer technique has been disclosed in Japanese Publication of Unexamined Patent Application (Tokukai) Sho63-183623. The duplicate record technique of a magnetized pattern using the magnetic transfer technique was originally developed as a method for copying the contents of a videotape. This technique is explained in detail in C. D. Mee and E. D. Daniel, xe2x80x9cMagnetic Recordingxe2x80x9d, Vol. 3, Chapter 2, p94-105, for example. The method disclosed in Tokukai Sho63-183623 applies the above duplication technique for videotape to the preformat writing of the tracking servo signal or other signals for a flexible disk.
Such a magnetic transfer technique may improve the productivity of the preformat writing. However, this technique is effective only for media such as flexible disks that have a small coercive force and a low areal record density. It is not effective for today""s hard disks, which have a large coercive force and a high areal record density in the order of several hundred megabits to gigabit.
In the magnetic transfer technique, an alternating bias magnetic field has to be applied, whose amplitude is approximately 1.5 times the coercive force of the target (slave) disk to ensure high transfer efficiency. The coercive force of the master disk should be more than three times of that of the slave disk, so that the master information, ie. a magnetized pattern in the master disk, is not erased by the alternating bias magnetic field. Today""s high-density hard disk media have a coercive force of 120-200 kA/m to enable a high-areal recording density. It is estimated that the coercive force will reach 250-350 kA/m for an areal record density of 10-gigabit order in the future. This means that a master disk should have a very large coercive force of 360-600 kA/m at present and 750-1050 kA/m in the future.
It is difficult to realize such a large coercive force for a master disk from the standpoint of a magnetic material. In addition, master information cannot be written into a master disk having such a large coercive force by any current magnetic recording method. Therefore, considering a possible coercive force for a master disk in the current magnetic transfer technique, the coercive force of the slave disk inevitably has an upper limit.
In the above-mentioned magnetic transfer technique, it is possible to utilize a thermo-magnetic transfer technique, where instead of applying the alternating bias magnetic field to the slave disk, the slave disk is heated to the temperature near to the Curie temperature for eliminating spontaneous magnetization. However, in that case, the Curie temperature of the slave disk should be much lower than that of the master disk. High coercive force magnetic film composed of Co group materials used for a high density magnetic record medium has a relatively high Curie temperature, so it is difficult to realize the characteristics required of the master disk and the slave disk for the thermo-magnetic transfer. Therefore, this preformat writing with a magnetic transfer technique cannot be a substantial solution for the before-mentioned problems.
Another solution for these problems is a pre-embossed disk technique disclosed in Publication of Japanese Unexamined Patent Application (Tokukai) Hei7-153060 (corresponding to U.S. Pat. No. 5,585,989 and European laid open patent application No. 655,734). In this technique, an embossed pattern corresponding to a tracking servo signal, address, clock signal and/or other signals is formed on a surface of the disk substrate by a stamper, and a magnetic film is formed on the substrate. This technique can be an effective solution for the before-mentioned problems. However, the embossed pattern on the disk surface may influence the head""s flying float performance (or contact state in the case of contact writing) when writing or reading, so that interface performance between the head and medium may be problematic. In addition, the substrate processed by the stamper is usually a polymer material (plastic), so it cannot be heated when forming the magnetic film for ensuring medium properties, and thus a necessary S/N ratio cannot be ensured.
As mentioned above, a truly effective solution of the before-mentioned two problems, which does not sacrifice other important performance such as the medium S/N ratio or the head-medium interface, has not been found yet.
Considering the above problems, the present invention provides a method and apparatus for improving the productivity of the preformat writing and the sharpness of the magnetic transition at edges of a track where the preformat is written, without sacrificing other important performance criteria such as the S/N ratio or the head-medium interface.
A method for writing a master information signal into a magnetic record medium according to the present invention uses a master information carrier comprising a substrate; an embossed pattern corresponding to the master information signal formed on the substrate; and a ferromagnetic material that forms at least the surface of the protruding portion of the embossed pattern. The surface of this master information carrier contacts with a surface of a target magnetic record medium having a sheet or disk shape, whose surface has a ferromagnetic thin film or coating. Thus, a magnetized pattern corresponding to the embossed pattern on the surface of the master information carrier is recorded into the magnetic record medium.
It is preferable that the ferromagnetic material forming the surface of the protruding portion is a soft magnetic material. Alternatively, it can be a hard or semihard magnetic material whose coercive force is less than 40 kA/m in the in-plane or perpendicular direction of the substrate.
It is more preferable to apply a direct (i.e., not alternating) magnetic field for exciting the ferromagnetic material forming the surface of the protruding portion, or an alternating magnetic field for assisting the writing of the magnetizing pattern, when the surface of the master information carrier contacts with the surface of the magnetic record medium.
According to the above-mentioned method of the present invention, a leakage flux is generated from the ferromagnetic material at the protruding portion of the surface of the master information carrier when the ferromagnetic material is magnetized in one direction. This leakage flux performs writing of the magnetized pattern corresponding to the embossed pattern of the master information carrier into the magnetic record medium. Thus, the preformat writing of the tracking servo signal, address signal, clock signal and other signals is achieved by using the embossed pattern formed on the surface of the master information carrier, corresponding to the information signal.
The writing method of the present invention utilizes a leakage magnetic field generated from the ferromagnetic material at the protruding portion due to the change of the magnetic reluctance through the embossed pattern Therefore, the writing mechanism is the same as a conventional magnetic record utilizing a leakage magnetic field generated from a gap of the magnetic head. However, in the writing method of the present invention, the master information of the whole plane of the master information carrier is written into the magnetic record medium at one time without relative movement between the master information carrier and the record medium. This characteristic point differs from the writing with magnetic head in the prior art, in which the head and the record medium move relative to each other. This characteristic point of the present invention provides an effective solution for the previously mentioned two problems, as follows.
First, the time needed for the preformat writing is substantially short compared with the prior art using a magnetic head. In addition, an expensive servo-tracking writer is not necessary for precise position control of the magnetic head. Therefore, the present invention can improve the productivity of the preformat writing and reduce production costs.
Secondly, a space gap between the master information carrier and the magnetic record medium can be minimized, since relative movement between them is not required for writing the information signal. In addition, the leakage magnetic field for writing does not diffuse, while it diffuses fringing over the record track width in the prior art using a magnetic head due to a pole shape of the magnetic head. Thus the magnetic transition at edges of a track into which the preformat data is written has sharpness compared with the writing with a magnetic head. This ensures a precise tracking of a head in reading data signals from the magnetic record medium.
Furthermore, the method of the present invention does not require the limitation of a structure or magnetic performance of the magnetic record medium in which the master information is written, differently from the magnetic transfer technique disclosed in Tokukai Sho63-183623 or the pre-embossed disk technique disclosed in Tokukai Hei7-153060 as previously stated.
For example, in the magnetic transfer technique disclosed in Tokukai Sho63-183623, the master disk requires a substantially high record resolution, since the master disk itself is a magnetic record medium having master information as a magnetization pattern of the master disk Consequently, the magnetic flux density and the film thickness cannot be enhanced sufficiently for enlarging magnetic field intensity for magnetic transfer. In addition, a gradient of the magnetic field for the magnetic transfer becomes small in the magnetic transition area since demagnetization occurs due to the repelling poles of the di-bit. To ensure a sufficient magnetic transfer efficiency with such a weak magnetic field for magnetic transfer, an alternating bias magnetic field is applied, which has an intensity of approximately 1.5 times of a coercive force of the target (slave) record disk. Therefore, this magnetic transfer technique can be applied only to a flexible disk or other medium with low record density since the coercive force is limited as previously stated.
On the contrary, the master information carrier of the present invention has the master information as an embossed pattern, and a leakage magnetic field, which is generated from a ferromagnetic material at a protruding portion of the embossed pattern due to a change of a magnetic reluctance through the embossed pattern, performs the magnetic recording of the master information. The master information carrier does not require a high resolution as the magnetic record medium, though it is required for the master disk in the magnetic transfer technique. Therefore a magnetic flux density and a thickness of the ferromagnetic material that forms the protruding portion of the surface of the master information carrier can be as large as the magnetic record head used in the prior art, so that a sharp and large recording magnetic field can be obtained similarly to a magnetic record head. Thus, a sufficient writing ability can be obtained for any magnetic record medium, including a usual flexible disk and hard disk and a record medium with a high coercive force for a gigabit recording in the future.
The pre-embossed disk technique disclosed in Tokukai Hei7-153060 may require a sacrifice of the medium S/N ratio relating to a substrate temperature at film formation process and the head-medium interface performance relating to a head floating performance (or contacting state), since the substrate material and shape of the disk are restricted as previously explained. On the contrary, the writing method of the present invention has no limitation about the substrate material and surface shape of the disk to be written for the preformat.
As mentioned above, the writing method of the present invention provides an essential solution for the previously mentioned two problems without sacrificing other important performances such as the medium S/N ratio and interface performance.
It is also effective in this writing method to apply an alternating and decaying bias magnetic field for obtaining higher writing efficiency. In this case, there is no possibility of erasing a master information by the alternating magnetic field or other external magnetic field since the master information is formed by the embossing pattern in the master information carrier of the present invention differently from the master information written as a magnetization pattern in the magnetic transfer technique. Therefore, the coercive force of the ferromagnetic material that forms the protruding portion of the surface of the master information carrier has no limitation. The ferromagnetic material is not limited to a material with high coercive force, but can be selected from a variety of materials such as a semihard magnetic material or a soft magnetic material as long as the material can generate sufficient magnetic field for writing the master information into a magnetic record medium.
In the writing method of the present invention, the ferromagnetic material that forms the protruding portion of the surface of the master information carrier should be magnetized in one direction to generate a magnetic field for writing. Therefore, if the semihard or soft magnetic material used as the ferromagnetic material cannot generate a stable one-way magnetization, or if a large amplitude of alternating bias magnetic field is applied, it is necessary to apply a direct exciting field for exciting the ferromagnetic material and generating an adequate intensity of magnetic field for writing. This direct (not alternating) magnetic field corresponds to the magnetic field generated by a drive current in wring coils of a magnetic head.
As mentioned above, the present invention provides a method for preformat writing of a tracking servo signal, address signal, clock signal or other signals into a magnetic record medium, especially a disk medium such as a hard disk or a large capacity flexible disk, with substantially high productivity and low cost.
The present invention also provides more precise tracking for a higher track density than in the prior art.
The present invention provides an essential solution for the previously stated problems in the prior art without sacrificing any important performances such as a medium S/N ratio or a head-medium interface performance. Thus, the present invention will be an important technology for a magnetic record medium with a high record density of gigabit order and above in the future.